6.15.2.2 Tropospheric ozone

The radiative forcings associated with future tropospheric O3 increases
are calculated on the basis of the O3 changes calculated by Chapter
4 and presented in Appendix II, Table
II.2.5 for the various SRES scenarios. The mean forcing per DU estimated
from the various models and given in Table 6.3 (i.e.,
0.042 Wm-2/DU) is used to derive these future forcings. Most scenarios
lead to increases in the abundances of tropospheric O3 and consequently
to positive radiative forcings in 2050 and 2100. Scenarios A1fi, A2, and A2p
provide the maximum tropospheric O3 forcings reaching 0.89 Wm-2
in 2100. Only scenario B1 predicts a decrease in tropospheric O3
and a negative forcing of -0.16 Wm-2 in 2100.

6.15.2.3 Aerosol direct effect

The direct radiative forcing due to anthropogenic sulphate, black carbon (BC),
and organic carbon (OC) aerosols are assessed using the scenarios described
in Chapter 5, Section 5.5.3
and the column burdens presented in Appendix II, Tables
II.2.7, II.2.8 and II.2.9 respectively.
Scenarios for mineral dust are not considered here, as there is no “best
estimate” available (Sections 6.7.6 and 6.13).
For each aerosol species, the ratio of the column burdens for the particular
scenario to that of the year 2000 is multiplied by the “best estimate”
of the present day radiative forcing. The year 2000 “best estimate”
radiative forcing is then subtracted to give the radiative forcing for the period
from the year 2000 to the date of the scenario. Estimates of the direct radiative
forcing for each of the aerosol species are given in Table
6.15.

The uncertainty associated with these estimates of the direct radiative forcing
is necessarily higher than the estimates for the year 2000. This is because
estimates for the year 2000 are constrained as best as possible to observations.
Additionally, the simplified approach applied here does not account for changes
in the spatial pattern of the global distribution of the aerosol species that
may arise from changes in the geographic distribution of emissions. The uncertainty
is therefore estimated as a factor of three for the radiative forcing by sulphate
aerosols, and a factor of four for the radiative forcing by BC and OC aerosols.

For sulphate aerosols, only scenarios A1fi, A2, and A2p show a negative direct
radiative forcing for the period 2000 to 2050, with all scenarios showing positive
radiative forcings over the period 2000 to 2100. This contrasts with the IS92a
scenario which predicted an increasingly negative radiative forcing due to the
higher column burden of sulphate. For BC aerosols the majority of the scenarios
show an increasingly positive direct radiative forcing in 2050 and 2100 (except
for B1 and B1p). For OC aerosols, the opposite is true, with the majority of
scenarios showing an increasingly negative direct radiative forcing in 2050
and 2100 (except for B1 and B1p). The direct aerosol radiative forcing evolution
due to sulphate, black and organic carbon aerosols taken together varies in
sign for the different scenarios. The magnitudes are much smaller than that
for CO2. As with the trace gases, there is considerable variation amongst the
different scenarios.

Table 6.15. Direct aerosol radiative forcings (Wm-2)
estimated as an average of different models for the IPCC SRES scenarios
described in Chapter 5. The burdens used in calculating
the radiative forcings are given in Appendix II, Tables
II.2.7, II.2.8 and II.2.9,
the radiative forcings presented here are from the year 2000 to the date
of the scenario. They may be added to the present day forcings given in
Section 6.7 (0.4 Wm-2 for sulphate aerosols,
+0.4 Wm-2 assumed for BC aerosols (from fossil fuel and biomass
burning), and -0.5 Wm-2 assumed for OC aerosols (from fossil
fuel and biomass burning)) to obtain the radiative forcings from pre-industrial
times.